The present invention relates to a group of metal magnetic nanoparticles and a method for producing the same. More particularly, the present invention is concerned with a group of metal magnetic nanoparticles having excellent oxidation resistance and excellent dispersibility and a method for producing the group of metal magnetic nanoparticles in high yield.
Magnetic nanoparticles have drawn attention as essential materials for achieving a variety of next-generation nanotechnology devices, such as high-density magnetic recording media, radio-frequency electromagnetic wave shields, new immunological test systems, and drug delivery systems. For the application of the magnetic nanoparticles to these devices, the development of a method for synthesizing a group of magnetic nanoparticles having controlled particle form, particle diameter, and particle size distribution is required. Particularly, the development of a method for synthesizing uniform nanoparticles of a magnetic material comprised of a metal or an alloy having a large magnetic moment is desired.
With respect to the method for synthesizing a group of magnetic nanoparticles, it is known that a liquid-phase synthesis method can control the particle form or particle size distribution, and liquid-phase synthesis methods for groups of nanoparticles comprised of Fe, Co, Ni, or an alloy thereof have been reported. See, Japanese Patent Application Publication No. 2000-54012; Japanese National Patent Application Publication of Translated Version (kohyo) No. 2003-515921; S. J. Park, S. Kim, S. Lee, Z. G. Khim, K. Char, and T. Hyeon, J. Am. Chem. Soc. 122, 8581 (2000); S. Sun and C. B. Murray, J. Appl. Phys. 85, 4325 (1999); and V. F. Puntes, K. M. Krishan, and A. P. Alivisatos, Science 291, 2115 (2001).
In addition, it has been reported that a particle growth method utilizing a noble metal atom cluster of Pt or Ag can produce particles having an extremely reduced particle diameter. See, G. Viau, P. Toneguzzo, A. Pierrard, O. Acher, F. Fievet-Vincent, and F. Fievet, Scripta mater. 44, 2263 (2001); D. Farrell, S. A. Majetich, and J. P. Wilcoxon, J. Phys. Chem. B 107, 11022 (2003); and C. Luna, M. P. Morales, C. J. Serna, and M. Vazquez, Materials Science and Engineering C 23, 1129 (2003).
However, the groups of metal magnetic nanoparticles obtained by the above synthesis methods have a problem in that the nanoparticles have poor chemical stability and hence are oxidized in air as a period of time lapses, so that their magnetic moment is gradually lowered. Therefore, when the group of nanoparticles which deteriorates with time is incorporated into a system, the resultant system has only limited reliability.
Further, the above synthesis methods have a problem in that part of or most of the group of nanoparticles suffers irreversible aggregation to cause settlings. The group of nanoparticles, which undergoes irreversible aggregation, is difficult to apply to various systems.